Method for reducing stress in epitaxial growth

ABSTRACT

A device and method for making the same are disclosed. The device includes a substrate having a first TEC, a stress relief layer overlying the substrate, and crystalline cap layer. The crystalline cap layer overlies the stress relief layer. The cap layer has a second TEC different from the first TEC. The stress relief layer includes an amorphous material that relieves stress between the crystalline substrate and the cap layer arising from differences in the first and second TECs at a growth temperature at which layers are grown epitaxially on the cap layer. The device can be used to construct various semiconductor devices including GaN LEDs that are fabricated on silicon or SiC wafers. The stress relief layer is generated by converting a layer of precursor material on the substrate after the cap layer has been grown to a stress-relief layer.

BACKGROUND OF THE INVENTION

A number of semiconductor devices are fabricated by epitaxially growinga number of semiconductor layers on a substrate. For example, one classof light emitting diodes (LEDs) is constructed by growing a number ofepitaxially grown layers of GaN semiconductors on a substrate. The yieldof devices from the fabrication process is reduced by defects in theepitaxially grown layers. One source of defects is the mismatch in thethermal expansion coefficients (TECs) between the epitaxially grownlayers and the substrate. In the case of GaN semiconductors grown onsapphire, significant mismatches between both the thermal expansioncoefficients and the lattice constants exist.

The mismatch is even greater for GaN semiconductor layers grown onsilicon. As a result, the epitaxially grown layers tend to crack whenthe substrate and layers are cooled from the growth temperature. Inaddition, the GaN layers tend to bow during the growth process due tothe thermal mismatch. This bowing interferes with the uniformity of thelayers across the wafer.

Since silicon wafers offer significant advantages over sapphire wafers,a growth technique that reduces the stress caused by the TEC mismatchbetween the GaN based layers and the underlying substrate is needed.

SUMMARY

The invention includes a device and method for making the same. Thedevice includes a substrate having a first TEC, a stress relief layeroverlying the substrate, and crystalline cap layer. The crystalline caplayer overlies the stress relief layer. The cap layer has a second TECdifferent from the first TEC. The stress relief layer includes anamorphous material that relieves stress between the crystallinesubstrate and the cap layer arising from differences in the first andsecond TECs at a growth temperature at which layers are grownepitaxially on the cap layer. The device can be used to constructvarious semiconductor devices including GaN LEDs that are fabricated onsilicon or SiC wafers.

The stress relief layer is generated by depositing a precursor materialon the substrate. A layer of semiconductor material is epitaxially grownon precursor material. The precursor material is then converted tostress relief material that relieves stress between the substrate andsemiconductor layers arising from differences in the first and secondTECs at a growth temperature at which layers are grown epitaxially onthe first semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-sectional views of a growth substrate accordingto one embodiment of the present invention.

FIG. 3 illustrates an LED according to the present invention that isconstructed on a growth substrate according to one embodiment of thepresent invention.

FIG. 4 is a cross-sectional view of one LED according to one embodimentof the present invention after the contacts have been provided.

FIG. 5 is a cross-sectional view of the LED wafer shown in FIG. 4 aftera second substrate has been attached.

FIG. 6 is a cross-sectional view of an LED utilizing an upside downconfiguration.

DETAILED DESCRIPTION

The present invention utilizes an stress relief layer between the growthsubstrate and the epitaxially grown layers. In one aspect of theinvention, the stress relief layer liquefies or becomes pliant duringthe epitaxial growth process, and hence, relieves any stress caused bythe thermal mismatch between the substrate and the epitaxially grownlayers to be relieved. When the epitaxially grown layers are cooled, thestress relief layer solidifies.

Refer now to FIGS. 1 and 2, which are cross-sectional views of a growthsubstrate according to one embodiment of the present invention. Growthsubstrate 20 is constructed on a silicon wafer 21. An AlN buffer layer22 is grown on top of substrate 21 to compensate for the differences inlattice constants between the materials of the GaN family and silicon.Next, an InGaN layer 23 is grown on top of layer 22. Finally, a GaN“cap” layer 24 is grown on top of layer 23. The cap layer provides alattice structure on which subsequent GaN family members can be grownepitaxially after layer 23 has been converted to a metallic layer.

One aspect of the present invention is based on the observation thatInGaN can be decomposed into an alloy of In and Ga by exposing the layerto high temperature or annealing the layer. Hence, after the conversion,the growth substrate includes a metal layer 25 that is sandwichedbetween buffer layer 22 and cap layer 24. The melting point of layer 25is below the epitaxial growth temperature for subsequent layers of GaNfamily member layers. Hence, these layers will be grown on a liquidmetal layer that prevents the difference in TEC between the silicon andthe GaN layers from inducing significant stress in either layer.

Refer now to FIG. 3, which illustrates an LED according to the presentinvention that is constructed on a growth substrate according to oneembodiment of the present invention. LED wafer 30 is constructed byepitaxially growing layers of GaN on a growth substrate 31 that issimilar to those described above. The LED layers typically include ann-GAN layer 32, an active layer 33, and a p-GaN layer 34. While LEDwafer 30 is described in terms of these three layers, it is to beunderstood that each of these layers may include a number of sublayershaving different compositions. Furthermore, while LED wafer 30 isdescribed in terms of GaN layers, it is to be understood that theselayers may be formed of a materials from the GaN family of materials.For the purposes of this discussion, the GaN family of materials isdefined to be all alloy compositions of GaN, InN and AlN.

To complete the construction of LED wafer 30, power contacts must beprovided to layers 32 and 34 for each of the individual LEDs into whichLED wafer 30 is be divided. Referring now to FIG. 4 which is across-sectional view of one LED according to one embodiment of thepresent invention after the contacts have been provided. The contact tothe p-GaN layer 43 may include a current spreading layer 42 to improvethe uniformity of the current flow through the p-GaN layer. If light isto be extracted through the p-GaN layer, spreading layer 42 must be atransparent material such as indium tin oxide. If light is to beextracted through substrate 21, current spreading layer 42 can be aminor constructed from a layer of silver. The second contact 41 to then-GaN layer is deposited in an etched trench that terminates on then-GaN layer.

In some applications, it is advantageous to remove substrate 21. Forexample, if substrate 21 is a silicon substrate, the blue lightgenerated in the active layer by a GaN LED will be absorbed in thesubstrate. In one aspect of the present invention, metal layer 25 isused to remove substrate 21 by heating the LED structure to atemperature at which metal layer 25 melts. At this point, substrate 21and buffer layer 22 can be detached from layer 24. Since the remaininglayers are only a few microns thick, these layers must first be attachedto a second substrate before substrate 21 is removed. Refer now to FIG.5, which is a cross-sectional view of LED wafer 30 shown in FIG. 4 aftera second substrate has been attached. Carrier substrate 44 is bonded towafer 30 via an adhesive layer 45. The adhesive layer can be any layerthat will withstand the heating of combined structures needed to liquefymetal layer 25.

An optional reflective layer 46 can be deposited on the upper surface ofthe p-GaN layer so that the light is extracted through the bottomsurface of layer 24 after substrate 21 and layers 22 and 25 have beenremoved. The reflective layer can also provide a contact and currentspreading function for powering the p-GaN layer. Referring now to FIG.6, which is a cross-sectional view of an LED utilizing this upside downconfiguration. In this embodiment, light is extracted through the n-GaNlayer 24. Contact 48 connects to this layer which acts as a currentspreading layer for the underlying n-GaN layer 32 of the LED. Contact 47is connected to mirror layer 46 by a trench cut in the LED layeredstructure. Mirror layer 46 provides the current spreading function forthe p-GaN layer.

The present invention has been described in terms of an InGaN layer thatis deposited on a buffer layer having a suitable lattice constant andthen converted to a metallic layer by heating after a subsequent GaN caplayer has been deposited. This results in a crystalline growth substratehaving a buried metallic layer. The GaN cap layer presents a surface onwhich subsequent layers from the GaN family of materials can beepitaxially grown without substantially reduced stresses resulting fromdifferences in the thermal coefficients of expansion between the GaNfamily materials and the underlying substrate.

The teachings of the present invention can be applied to otherepitaxially grown systems in which the differences between the thermalcoefficients of expansion between two layers present significantproblems. The method requires that a stress relief layer having twoproperties be grown between the layers in question. First, the stressrelief layer must have a precursor with a lattice constant that iscompatible with the lattice constants of the two layers in question andon which the next layer can be epitaxially grown before the precursormaterial is converted to a layer that will provide stress relief duringthe subsequent epitaxial growth. Second the precursor must beconvertible to a material that will relieve the stress between the firstand second layers at the growth temperature of the second and remaininglayers. In the examples discussed above, the stress relief layer is ametal that is in the molten state at the growth temperature in question.

In the above-described embodiments, the cap layer was different than thefirst layer of the light emitting device that was grown on the growthsubstrate. However, the first layer of semiconductor material of thelight emitting device could provide the function of the cap layer. Inthis case, conversion of stress relief layer needs to be done after thefirst semiconductor layer is grown. A separate cap layer has theadvantage of being a much thinner layer than the conventional firstsemiconductor layer, and hence, is less affected by the thermal stresscaused by the differences in TECs during the growth of the cap layerprior to the precursor material being converted to the stress relieflayer.

The above-described embodiments of the present invention have beenprovided to illustrate various aspects of the invention. However, it isto be understood that different aspects of the present invention thatare shown in different specific embodiments can be combined to provideother embodiments of the present invention. In addition, variousmodifications to the present invention will become apparent from theforegoing description and accompanying drawings. Accordingly, thepresent invention is to be limited solely by the scope of the followingclaims.

What is claimed is:
 1. A method for fabricating a semiconductor device,said method comprising: depositing a precursor stress relief layer on asubstrate characterized by a first TEC; epitaxially depositing a firstsemiconductor layer on said precursor stress relief layer, said firstsemiconductor layer being characterized by a second TEC, different fromsaid first TEC; and converting said precursor stress relief layer to astress relief layer comprising a stress relief material that relievesstress between said substrate and said first semiconductor layer arisingfrom differences in said first and second TECs at a growth temperatureat which layers are grown epitaxially on said first semiconductor layer,said stress relief material being non-crystalline at said growthtemperature.
 2. The method of claim 1 wherein precursor stress relieflayer comprises a material from the GaN family of materials that isconverted to said stress relief material.
 3. The method of claim 2wherein said precursor material is converted to said stress reliefmaterial by heating said precursor material.
 4. The method of claim 1wherein said precursor material comprises InGaN and wherein said stressrelief material comprises an alloy of In and Ga, said alloy being in aliquid state at said growth temperature.
 5. The method of claim 1further comprising epitaxially growing a light emitting device on saidfirst semiconductor layer.
 6. The method of claim 5 wherein said lightemitting device has a TEC substantially equal to said second TEC.
 7. Themethod of claim 5 wherein said light emitting device is a GaN LED andwherein said substrate comprises a silicon wafer.
 8. The method of claim7 further comprising removing said silicon wafer by heating saidsemiconductor device to a temperature at which said stress relief layeris in a liquid state.
 9. A method for fabricating a semiconductordevice, comprising: depositing a precursor stress relief layer on asubstrate, the precursor stress relief layer having a first TEC;epitaxially depositing a first semiconductor layer on said precursorstress relief layer, said first semiconductor layer having a second TECdifferent from said first TEC; and converting said precursor stressrelief layer to a stress relief layer comprising a stress reliefmaterial that relieves stress between said substrate and said firstsemiconductor layer arising from differences in said first and secondTECs by heating said precursor material at a growth temperature at whichlayers are grown epitaxially on said first semiconductor layer, saidstress relief material being non-crystalline at said growth temperature.10. The method of claim 9, wherein precursor stress relief layercomprises a material from the GaN family of materials that is convertedto said stress relief material.
 11. The method of claim 9, wherein saidprecursor material comprises InGaN and wherein said stress reliefmaterial comprises an alloy of In and Ga, said alloy being in a liquidstate at said growth temperature.
 12. The method of claim 9, furthercomprising epitaxially growing a light emitting device on said firstsemiconductor layer.
 13. The method of claim 12, wherein said lightemitting device has a TEC substantially equal to said second TEC. 14.The method of claim 12, wherein said light emitting device is a GaN LEDand wherein said substrate comprises a silicon wafer.
 15. The method ofclaim 14, further comprising removing said silicon wafer by heating saidsemiconductor device to a temperature at which said stress relief layeris in a liquid state.